Since we are **chemical** engineers, every now and then we
actually deal with chemicals!

In this lecture we will remind you of a bunch of things that you should have learned in chemistry about compositions and chemical "amounts".

DEFINITION:

**Atomic weight** is a measure of the mass
of an atom (C^{12} -> 12 units)

DEFINITION:

**Molecular weight** is the sum of the
atomic weights of the atoms that make up a compound.

DEFINITION:

A **mole (or gram-mol)** is the amount of
a species (atom **or** molecule) whose mass is numerically
equivalent to its molecular weight. (For example 12 grams of
C^{12} = 1 gram-mol of C^{12})

NOTE:

Molecular (atomic) weight can be used as a conversion factor between the number of X-moles and the mass in X units!

EXERCISE:

How do you convert 1 lb-mol of H_{2}O
to gram-moles of H_{2}O?

Compositions as fractions:

NOTE:

Percent = parts per hundred. We may also use parts
per million (ppm) or parts per billion (ppb). (Purely for
simplicity in describing dilute solutions, so that we need not say
0.0001% or 1.0e^{-7}%)

OUTCOME:

Calculate mass (mole) fractions from mole (mass) fractions for a mixture

TEST YOURSELF!

On a humid day, air may have a composition like
this: O_{2}: 19%, N_{2}: 72%, and H_{2}O:
9.0% (all percents are by MASS). What is the composition in mole
fractions?